Herbst Michael F, Fransson Thomas
CERMICS, École des Ponts ParisTech, 6-8 Avenue Blaise Pascal, 77455 Marne-la-Vallée, France; Inria Paris, 75589 Paris Cedex 12, France; and Sorbonne Universitée, Institut des Sciences du Calcul et des Données, ISCD, 75005 Paris, France.
Interdisciplinary Center for Scientific Computing, Heidelberg University, 69120 Heidelberg, Germany and Fysikum, Stockholm University, Albanova, 10691 Stockholm, Sweden.
J Chem Phys. 2020 Aug 7;153(5):054114. doi: 10.1063/5.0013538.
For the calculation of core-excited states probed through X-ray absorption spectroscopy, the core-valence separation (CVS) scheme has become a vital tool. This approach allows us to target such states with high specificity, albeit introducing an error. We report the implementation of a post-processing step for CVS excitations obtained within the algebraic-diagrammatic construction scheme for the polarization propagator, which removes this error. Based on this, we provide a detailed analysis of the CVS scheme, identifying its accuracy to be dominated by an error balance between two neglected couplings, one between core and valence single excitations and the other between single and double core excitations. The selection of the basis set is shown to be vital for a proper description of both couplings, with tight polarizing functions being necessary for a good balance of errors. The CVS error is confirmed to be stable across multiple systems, with an element-specific spread for K-edge spectrum calculations of only about ±0.02 eV. A systematic lowering of the CVS error by 0.02 eV-0.03 eV is noted when considering excitations to extremely diffuse states, emulating ionization.
对于通过X射线吸收光谱探测的芯激发态的计算，芯价分离（CVS）方案已成为一种重要工具。这种方法使我们能够以高特异性靶向此类状态，尽管会引入误差。我们报告了在极化传播子的代数图示构造方案中获得的CVS激发的后处理步骤的实现，该步骤消除了此误差。基于此，我们对CVS方案进行了详细分析，确定其精度主要由两个被忽略的耦合之间的误差平衡决定，一个是芯与价单激发之间的耦合，另一个是单与双核激发之间的耦合。结果表明，基组的选择对于正确描述这两种耦合至关重要，紧密的极化函数对于良好的误差平衡是必要的。CVS误差在多个系统中被证实是稳定的，对于K边光谱计算，元素特异性的误差范围仅约为±0.02 eV。在考虑激发到极其弥散的状态（模拟电离）时，注意到CVS误差系统地降低了0.02 eV - 0.03 eV。
